The invention concerns a process for the removal of acid components and nitrogen oxide from the exhaust gas from industrial furnaces, by oxidizing the nitrogen oxide and subjecting the exhaust gas to a two-stage absorption employing aqueous ammonium salt solutions as absorption liquid, whereby the ammonium salt contained in the absorption liquid is obtained in the process itself through the employment of aqueous ammonia solution and are separated in connection with the gas treatment by means of evaporation of the absorption liquid.
In recent years a number of processes are known which serve the purpose of decreasing through suitable measures the environmental loading caused by the exhaust gases from industrial furnaces, particularly the exhaust gases from coal- and/or oil-burning power plants. In this regard, numerous suggestions are disclosed in the literature for the simultaneous separation of SO.sub.x and NO.sub.x from these exhaust gases. These processes employ herewith not only washing but also dry methods for the separation of these components. A process of the above-depicted type is described, for example, in "Chemie-Ingenieur-Technik", 57 (1985), pages 720-722. With these techniques, known as the Walter Simultaneous Processes, the exhaust gases, after utilization of a part of the heat content for spray drying of the product salts, are initially introduced to a desulfurization with aqueous ammonia solution. In the desulfurized gas, the nitrogen oxide is brought to a higher oxidation stage by means of addition of ozone and/or ozonized air. The higher oxidized nitrogen oxides are then washed in a second washing stage with aqueous ammonia solution. The concentrated salt solutions from both washing stages are after-oxidized with air, therewith only providing yet sulfate and nitrate, and thereafter introduced together to the spray drying.
Tests of these processes in large-scale employment have proven that the desulfurization leads to that degree of desulfurization required in practice only at a pH-value from 5 on up. These pH-values, however, lead to an increased NH.sub.3 -partial-pressure which favors the formation of aerosols of the ammonium salts. These aerosols, which exit in the form of a so-called white vapor from the chimney from behind the waste gas washing, likewise represent an environmental burden, which must be thwarted. Indeed, the separation of these aerosols requires the construction of special filters. This is, however no satisfactory answer to the problem, since on the one hand the construction of a suitable filter system necessitates considerable investment costs, and on the other hand, on account of high pressure loss in the filter, the operational costs of the plant are increased undesirably in extent. The nitrogen oxide washing reaches in these processes the required degree only when the oxidation of the nitrogen oxide follows up to the anhydride of nitric acid, N.sub.2 O.sub.5. In order to attain this degree of oxidation with the use of ozone as oxidation agent a mol ratio of ozone to NO.sub.x in the exhaust gas of at least 1.5 is required. This high ozone requirement signifies, however, high operational and investment costs.